I've been wondering if GNURadio (which is licensed under GPL) can be "integrated" with proprietary
code and distributed, for some value of "integrated".

I think this is something that may have to be clarified for researchers at universities, at least
those with "intellectual property" offices. This may also be of interest to companies that need a
relatively disconnected front end for signal processing, and may want to invest their signal
processing developer resources in an open source project.

Example:

Lets say you write a ATSC receiver, using the GNURadio signal processing blocks. This server program
transmits an MPEG2 transport stream through a TCP/IP socket and accepts commands through another
TCP/IP socket. Then you write a proprietary client program that connects to this ATSC receiver
server through the TCP/IP ports. The ATSC receiver would definitely have to licensed under GPL. What
about the client program? Would this depend on what type of API the ATSC server has?